mgr/dashboard: Downstream branding

mgr/dashboard: Downstream branding

Adds the downstream branding to the ceph dashboard.

- New sign in page
- New about dialog
- Changed color scheme
- Replaced logos

Update on 04.09.2020 - Fixed bz:1175169, bz:1175161
Update on 25.09.2020 - Fixed bz:1175120

Signed-off-by: Sebastian Krah <skrah@suse.com>
(cherry picked from commit 7563f3305a12f155c5d171e93434f1372b977ae9)

Conflicts:
  src/pybind/mgr/dashboard/frontend/src/app/shared/services/doc.service.ts
  src/pybind/mgr/dashboard/frontend/src/app/shared/services/doc.service.spec.ts
  src/pybind/mgr/dashboard/frontend/src/styles/vendor.overrides.scss
  src/pybind/mgr/dashboard/frontend/src/styles/vendor.variables.scss